88 CHEMICAL ENGINEERING of the belt and a troughing idler should be placed about 6 in. to the rear of the discharging lip of the loading chute to give added support to the belt. Where the carrying belt takes an abrupt change in direction, a straight-face bend pulley is substituted for a troughing idler at the point at which the bend occurs and the troughing idlers immediately preceding and following the bend pulley should be situated from it at distances somewhat less th,an the spacing of idlers on straight carrying sections of the conveyor. When belt conveyors exceed 25 ft. in length—they have been built up to 1,000 ft. in length—guide idlers mounted normally to the edges of the belt on the carrying side should be installed about every 25 ft. to prevent the belt from leaving the troughing idlers. Discharge of load occurs over the head-end pulley of the conveyor or may take place at any point along the conveyor if the carrying belt is carried over an elevated FIG. 1.—Standard three pulley belt-conveyor idlers. discharge pulley (straight) and then around a lower bend pulley (straight) situated slightly to the rear of the discharge pulley so that the carrying belt takes a reverse S-loop. Such discharge points may be permanently located, in which case they are termed "fixed dumps," or the discharge and bend pulleys may be mounted on a traveling carriage and shifted from place to place on a horizontal section of the conveyor, in which case the discharging carriage is termed a "tripper." Trippers may be of the hand-propelled type or the power for their movement can be secured from the traveling belt—automatic traveling trippers. By a system of stops, automatic trippers may be made to travel automatically back and forth between designated points, distributing the belt load uniformly over their discharging path. Loading and discharging belt conveyors should be through the agency of chutes so inclined that the velocity of the material leaving the chute should be as nearly as possible the same as the speed of the conveyor belt, and in the same direction. The driving pulley, which should be heavily crowned in order to maintain alignment of belt, is advisably the head pulley of the conveyor, in order that the tension side of the belt may carry the load, but when this arrangement is not feasible the drive may be located at the tail end of the conveyor or at any poin along its lower run by looping the return belt much as the carrying belt is looped for a discharge point. For unusually long belt conveyors, two driving pulleys geared together are employed, in which case the driving section of the belt forms a double loop. A stall more powerful drive is secured by covering the driving pulleys with lagging to increase their grip on the conveyor belt. Typical types of conveyor drives are shown in Fig, 2.